Bacterial cells of Pseudomonas fluorescens TN5 (Pf) were immobilized behind a dialysis membrane on the surface of a carbon paste electrode. The electrode produced an anodic current in a buffer solution containing nicotinic acid in the presence of phenazine methosulfate. The anodic current was attributable to the oxidative hydroxylation of nicotinic acid by the catalytic action of the bacterium, in which PMS functioned as an electron acceptor in the catalytic reaction and as an electron transfer mediator between the immobilized bacterial cells and the electrode. Such compounds as 1-methoxy-5-methylphenazinium methylsulfate, 2,6-dichlorophenolindophenol, p-benzoquinone (BQ), ferricyanide and ferrocene carboxylic acid were also effective as mediators in the reaction. When a Pf-modified electrode was constructed with a carbon paste electrode containing BQ, the electrode showed a current response to nicotinic acid in the absence of a mediator in solution. The current response of the Pf-modified BQ-containing electrode was highly selective to nicotinic acid. The bioelectrocatalysis behavior of the electrode is investigated in some detail, and the results are interpreted on the basis of a simple model of bioelectrocatalysis at a biocatalyst-modified electrode.